Ed Miliband, leader of the Labour party, addresses the Scottish Labour party conference at Eden Court in Inverness on 19 April. Photograph: David Cheskin/PA

There are two possible reasons why no one seems to know what Ed Miliband would do if he became prime minister: either he is keeping it a secret, or he has no idea himself.

Miliband's entourage insists there is a plan, that he proposes a once-in-a-generation shift in the way Britain is governed. There will be a break from the consensus that prevailed before the financial crisis as profound as Margaret Thatcher's rupture from the reigning assumptions of the 1970s. Devising this new, morally responsible capitalism is a project – under the "one nation" Labour rubric – that will come to fruition in the period before the election.

However, Miliband's airy rhetoric leaves gaping holes for the Tories to fill with their own version of what a Labour government is about: bankrupting the country, ramping up debt, subsidising dissolute scroungers, opening the borders to mass immigration.

The message of a Conservative campaign is already clear: "The road has been harder than we hoped (for which, blame Europe), but we are on the right track. Don't give the keys back to the maniacs who drove us off the cliff." That dish will be seasoned with salty personal attacks on Miliband, who, despite incremental improvements in presentation, is still seen in Number 10 as a laughable candidate for national captaincy.

Labour strategists draw confidence from two things. First, they doubt the economy will recover enough for George Osborne to boast his plan has worked. Second, they have studied the lie of marginal seats and worked out that it is hard for David Cameron just to repeat his performance in 2010 – an open-goal election for the Tories. And he didn't get a majority even then.

The Labour core vote is energised by old anti-Tory passions and bolstered by aggrieved leftish refugees. So, in theory, Miliband could sneak into Downing Street with 35% of the vote and without needing to woo many Conservative defectors. That calculation breeds caution. It prioritises the brittle unity of an anti-austerity coalition held together by deferral of tricky policy choices. It counsels against pledges of fiscal discipline, statements of budget priorities and candour about cuts. It stifles discussion of ways to improve public services when hosing them down with money isn't an option. What do "one nation" Labour schools and hospitals look like if their budgets are frozen? To lead that conversation, Miliband would have to give a frank account of what he would resuscitate and what he leaves for dead in the reform agenda once known as Blairism – a subject certain to sow internal party strife.

It is easy to see the appeal for Miliband in dwelling on the abstract questions of how to remake capitalism from first principles. It is a task that necessarily precedes awkward choices about second, third and fourth principles. Working out what a Labour government should mean is a key phase of opposition, but it risks becoming a distraction from decisions about what that government would do. Miliband's defence is that there are two years in which to make that transition. Besides, there is so much turmoil in the economy that policies devised today might easily be obsolete tomorrow. The suspicion grows that all those big ideas are serving as not so much the foundations of a programme for government, but as an evasion of the practicalities. And if voters sense that Miliband is asking for power without knowing what to do with it, they won't give it to him.